Method for phosphatizing ferrous surfaces

ABSTRACT

Disclosed is a method of replenishing an aqueous acidic manganese phosphating bath. The main bath has a weight ratio of free to total P2O5 of 0.05 - 0.45:1 and the make up has a ratio of 0.5 to 1.0:1.

United States Patent 1191 Hansen et al.

[ Jan. 14, 1975 METHOD FOR PHOSPHATIZING FERROUS SURFACES [73] Assignee: Oxy Metal Finishing Corporation,

Warren, Mich.

22 Filed: Mar. 16, 1973 21 Appl. No.2 341,831

[52] U.S. Cl. 148/6.l5 R, 148/616 [51] Int. Cl. C231 7/10 [58] Field of Search 148/615 R, 6.15 Z, 6.16

[56] References Cited UNITED STATES PATENTS 2,293,716 8/1942 Darsey 148/615 2 2,500,673 3/1950 Gibson eta1.... 148/615 Z 3,450,578 6/1969 Siemund et al. 148/615 R 3,607,453 9/1971 Engesser et al. 148/615 Z Primary Examiner--Mayer Weinblatt Assistant Examiner1ohn Douglas Smith Attorney, Agent, or FirmArthur E. Kluegel [57] ABSTRACT Disclosed is a method of replenishing an aqueous acidic manganese phosphating bath. The main bath has a weight ratio of free to total P 0 of 0.05 0.45:1 and the make up has a ratio of 0.5 to 1.0:].

5 Claims, No Drawings METHOD FOR PHOSPHATIZING FERROUS SURFACES BACKGROUND OF THE INVENTION The invention pertains to an improved method for phosphatizing ferrous surfaces, with the aid of phosphate solutions based on manganese phosphate or manganese-iron phosphate.

It has long been known that steel can be coated with manganese phosphate or manganese-iron phosphate layers by bringing it into contact with aqueous acidic manganese phosphate solutions, which if desired may also contain divalent iron and/or oxidizing agents. The solutions contain sufficient free acid to bring the solution into chemical equilibrium with the layer-forming substance. This state corresponds to a certain equilibrium ratio of free P to total P 0 in the bath. This equilibrium acid ratio depends to some extent on the P 0 concentration, the presence of other solution components, and the working temperature, among other things. It is generally inadvisable to exceed the equilibrium concentration of free acid (free P 0 since to do so would have an unfavorable effect on the layer formation. Depending on the type of pretreatment, composition and the processing state of the material to be handled, the layers become thinner with increasing free acid; sometimes they also become more coarsely crystalline, and finally fail to form; instead, only tarnish colors and more extensive erosion upon pickling are noted. For this reason, the monitoring of the acid ratio during practical use of the manganese phosphate method is considered particularly important. Therefore, the recommendations advise the buffering of excess acid, e.g., with manganese carbonate. The aqueous manganese phosphate concentrates used for the initial batch and for makeup are adjusted as closely as possible to the equilibrium value of the working bath in free acid content. (See Handbuch der Galvanotechnik [Handbook of Electroplating Technology],

Munich 1969; ed. by H. W. Dettner and J. Elze, Volf 111, Chapter on Phosphation, p. 109; also see 0. L. Gilbert, Phosphating Materials and Processes; Rock Island Arsenal Laboratory Report No. 54-2906, PB 1 l I 726 [1954], e.g., pp. 49, 57, 58).

A method is also known in which an excessive amount of free acid is intentionally used in the bath, in order to produce very thin layers. However, these layers are only of practical use in specialcases, because of their very low weight per unit area (German Pat. No. 1,246,356).

SUMMARY OF THE INVENTION Surprisingly, it has now been found that while maintaining certain conditions in the working bath, and in the makeup, it is permissible to add more free P 0 in proportion to the total P 0 than that which would correspond to the phosphation equilibrium in the working bath, without disadvantageously affecting the layerforming properties of the bath. On the contrary, it was found that in this manner it was even possible to attain appreciable advantages, e. g., a distinct reduction in the sludge formed during phosphation and a reduction in the quantity of chemicals required for forming a given layer depth. The increased weight ratio of free P 0 to total P 0 also makes it possible to prepared makeup concentrates with increased chemical contents, thus also producing savings in expenditures for transportation, storage and the like.

DETAILED DESCRIPTION OF THE INVENTION The method in accordance with the invention for producing manganeseor iron-manganese-phosphate layers on steel in aqueous manganese phosphate or manganese-iron phosphate solutions is characterized in that the workpieces are brought into contact with aqueous bath solutions containing 1 to 35 g/l, preferably l to 24 g/l Mn; 0 to 30, preferably 0 to 29 g/l Fe II; S to g/l P 0 preferably 5 to 50 g/l P 0 0 to 80 g/l of a strongly acidic inorganic anion preferably 0 to 50 g/l N0 exhibit a point number between 15 and l50. preferably from 25 to I00, and in which the individual components are in the following weight ratios to one another: Fe(lI):Mn (0 to 10, preferably 0 to 9):l; MnzP O (0.02 to 2.5, preferably 0.02 to l.0):l; NO :P O (0 to 3, preferably 0 to 2): 1: free P O ztotal P 0 0.05 to 0.45, preferably 0.05 to 0.40):l. The point number is defined as the number of ml of N/l0 NaOH required to neutralize a 10 ml bath sample, using phenophthalein as indicator. The baths are supplemented according to the invention with MnzP O z- N0 in a weight ratio of (0.05 to 06, preferably 0.070.45):l:(0 to 1, preferably 0 to 0.9), wherein a weight ratio of free P O ztotal P O (0.5 to l, preferably 0.6 to l) is maintained. Particularly favorable relationships with reference to the possibility of concentrating the chemicals for makeup and the advantages described above are obtained when, in the makeup, the weight ratio of free P O :total P 0 amounts to (0.65-1 ):l. Preferably the ratio amounts to (0.7l ):I.

In the bath and/or makeup solution, the stronglyacidic inorganic anion may include one or more of N0 Cl, and 50}. While nitrate is preferred, it can be partially or completely replaced by Cl and/or 80,, wherein to each part by weight of N0 0.57 parts by weight of Cl and 0.77 parts by weight of SO, are used. Furthermore, in the makeup, the P 05 can be introduced partially or completely in the form of polyphosphoric acid or polyphosphate. The simultaneous use of Cl and/or 80., makes it possible to produce solutions containing more manganese without at the same time having to increase the nitrate content, which by way of its specific action affects the layer formation, the phosphation rate and the iron accumulation in the bath. The presence of poly-P 0 in the makeup concentrate makes it possible to produce stable makeup concentrates with high solids contents even at low values for the free P 0 to total P O The bath solutions in accordance with the invention can also contain additional components. Nickel and/or cobalt ions support the formation of layers on more difficult-to-attack materials such as those made of lowalloy steel. Similar effects are seen with simple and complex fluorides. By using stronger oxidizing agents such as chlorates, peroxides, bromates, and nitrites it is possible to limit the iron ll content, or to keep the baths free from iron (ll) even at high throughputs. The baths are preferably used in an immersion method at temperatures above 60C, generally between 80C and the boiling point.

The makeup solutions in accordance with the invention can contain, in addition to the principal components Mn, P 0 N0 (or Cl and/or 80,), small quantities of other additives such as nickel, cobalt, alkali, calcium and ammonium cations or anions from the group of simple and complex fluorides as well as special substances with accelerating effects. Essential for the procedure in accordance with the invention is the maintenance of the indicated ranges in the makeup for Mn, P N0 (or Cl and/or 80,), and for free P o ztotal P205. I

The free P 0 is determined as follows: ml of bath solution are titrated with 0.1 N NaOH to the endpoint of the first dissociation step of H PO Indicators such as dimethyl yellow or methyl orange can be used for indicating this endpoint. Each ml of consumed 0.1 N corresponds to 0.71 g/l of free P 0 The value for total P 0 can be determined according to the known methods of phosphate analysis, e.g., by precipitating the phosphate with ammonium molybdate and weighing the precipitate.

The calculation of the ratio for free P O :total P 0 can also be performed in accordance with statements in the Handbuch der Galvanotechnik (Handbook of Electroplating Technology), Vol. III, by W. Dettner and J. Elze, Carl Hanser Verlag, Munich, 1969, p. 90.

During the throughput of the material, phosphating chemicals are consumed in the formation of the layer and the precipitated sludge. The point number serves as a measure of the bath concentration. In order to return the point number to its original state after a certain amount of material has passed through or to keep it in the desired range, the bath is treated with an appropriate amount of makeup solution and mixed well. It is advisable to make the supplementation as often as possible, and in small portions.

The workpieces are freed from oils, fats, dust, rust and scale before phosphating by suitable means such as degreasing in organic solvents, acidic, neutral or alkaline cleaners, sandblasting, blasting with steel grains or corundum, or pickling in strong alkalies with the addition of complexing agents or in strong acids. In general, the workpiece is thoroughly rinsed with water between the cleaning of the metal surface and the phosphation. Especially after cleaning with alkalies and acids, the workpieces are pre-rinsed in an aqueous suspension of finely divided manganese phosphate, in order to promote the formation of particularly uniform, finecrystalline layers during the subsequent phosphation. The effect is particularly marked in the case of phosphation in baths containing iron (II) in addition to manganese. Manganese phosphate pre-rinses, in which a large portion of the manganese is present as hureaulite and/or contains additionally dissolved condensed phosphates, such as Na P O Na P O and the like, are preferred. To create the manganese phosphate or ironmanganese phosphate layers, the workpieces are generally brought into contact with the phosphation bath for 1 to 30 min., perferably by immersion. Then they are rinsed with water and treated according to the intended EXAMPLE l Panels and machine parts made of steel were dipdegreased in a highly alkaline, aqueous cleaner, then rinsed in water and pre-rinsed at room temperature in a solution containing 2 g/l finely dispersed manganese phosphate to which 2 g/l Na.,P O had been added. Then the parts were dip-phosphated for 10 min. at 92 to 98C in various manganese phosphate solutions,,

then rinsed and dried. The special experimental conditions and results are summarized in Table I.

Table 1 provides the following conclusions:

At practically identical layer weights, the quantity of sludge in the case of the method in accordance with the invention, methods C and B, amounted to only to of the quantity obtained in accordance with method A, which corresponds to the state of the art. Likewise, the consumption of chemicals in the case of B and C was substantially lower than in the case of A. This is partially due to the fact that the makeup concentrates B and C could be made substantially more concentrated than the makeup concentrate A. The overall concentration could be increased by a factor of 1.5, with a cold stability which was even improved compared to that of A.

EXAMPLE 2 In order to determine the influence of the composition of the makeup concentrate on the resistance to crystallization, a large number of solutions was made up and subjected to the cold test. Solutions which contained only H O, P 0 Mn and N0 will serve here as an example of the influence of the weight ratios of the solution components on the cold stability. Table 2 pres- TABLE 1Continued Experiment A Experiment B Experiment C (comparison) (invention) (invention) free P O,:ttal P205 0.25:1 0.33:1 0.25:1 makeup to a constant point number with a concentrate containing Mn (70) 5.53 5.82 2.93 P205 (70) 16.75 18.72 30.15 NO; 1.80 11.37 3.24 Ni 0.15 0.26 0.28 Mn:P O,,:NO 0.33:1:011 0.3 l :1 10.61 0.097:l:0.ll free P o ztotal P205 0.25:1 0.85:1 0.85:1 total concentration Mn Ni N0 P20, 24.23 36.17 36.60 beginning crystal formation at "C 1 5 9 bath composition after throughput of 2 m steel surface, per liter:

Mn (g/l) 7.3 9.0 5.0 Fe(ll) (g/l) 4.5 4.9 6.0 P 0, (g/l) 35.2 30.1 36.0 N0, (g/l) 4.1 4.0

Ni (g/l) not determined not determined not determined point number 74 77 76 FellzMn 0.62:1 0.5411 1.211 Mn:1 ,o 0.21:1 0.30:1 0.14:1 N05150:, 0.12:1 0.67:1 0.11:1 free 150.:101111 P205 0.24:1 0.27:1 0.22:1 layer weight (g/m) 7 6 8 sludge weight (g/m) l2 7 9 chemical consumption, concentrate (g/m) 48 26 28 TABLE 2 Total con- Weight ratios Mn:P. ,O -,:NO and free P O :total P 0 for centratlon Mn+P O +NO concentrates cold-stable to 0C 24 0.29:1:0; 0.25:1 0.48'.l:0.5; 0.33:1 0.66:1:09; 0.33:1 26 0.241110; 0.38:1 0.44:1:05; 0.45:1 0.62:1 :09; 0.44:1 30 0.17:1:0; 0.57:1 0.38:l:0.5; 0.58:1 0.57:1 :0.9; 0.56:] 34 0.11:1:0; 0.73:1 0.33:1:05; 0.73:1 0.52:1:0.9; 0.69:1 38 0.07:1:0; 0.80:1 0.27:l:0.5; 0.86:1 0.46:l:0.9; 0.85:1 42 0.06:1:0; 0.85:1 0.22:1:05; 1.00:1 0.40:1;09; 0.99:1

x: belonging to the total range of the invention x: belonging to the preferred range of the invention.

What iS Claimed i 2. The method of claim 1 wherein said makeup solul. In a metho of PhOSPhKIiOII Of Steel with an q tion comprises manganese and P 0 in a weight ratio of ous acidic manganese phosphate treating solution 0.05 0.6:1, respectively, and additionally contains a wherein the We ght ratiO of e P20 10 t a 205 in 45 strongly acidic inorganic anion in a weight ratio of said treating solution is between 0.05 and 0.45:1, the anion to P 0 not exceeding 1:1. improvement comprising replenishing said treating SO- 3. The method of claim 2 wherever the weight ratio lution with an aqueous makeup solution comprising of free to total P 0 in said makeup is 0.65 1:1. manganese and P 0 in a weight ratio between 0.05 and 4. The method of claim 2 wherever said weight rauo 0.6:1 and wherein the weight ratio of free to total P 0 of manganese to P 0 is 0.07 0.45: l. is between 0.5 and 1.011, inclusive of any neutralizing 5. The method of claim 1 wherever at least a portlon agent added. of the P 0 is present in the form of a polyphosphate. 

2. The method of claim 1 wherein said makeup solution comprises manganese and P2O5 in a weight ratio of 0.05 - 0.6:1, respectively, and additionally contains a strongly acidic inorganic anion in a weight ratio of anion to P2O5 not exceeding 1:1.
 3. The method of claim 2 wherever the weight ratio of free to total P2O5 in said makeup is 0.65 - 1:1.
 4. The method of claim 2 wherever said weight ratio of manganese to P2O5 is 0.07 - 0.45:1.
 5. The method of claim 1 wherever at least a portion of the P2O5 is present in the form of a polyphosphate. 